I remember years ago when I first got into the hobby visiting an Angle breeder at his house. He had a 55g with over 80 Angles that were the size of a half-dollar coin not including fins.
This was way before power filters or sumps so he was using an UGF. Having the 1-inch of fish per gallon rule already cemented into my skull, I was floored. I will never forget his answer when I asked, “how?”
“You can keep about as many as you want as long as you do enough water changes.” For him that was 20% every day. I still wouldn’t do it today, even if I had the time, but I know he did it for years.
The 1-inch of fish per gallon rule is still a good general guide, but it is very flawed. First, it outdates today’s technology. Second, it requires a lot of common since. A 1-inch neon tetra is obviously not the same as a 1-inch Puffer, which isn’t the same as a 1-inch Tiger Barb. Then you go into fish that are more sensitive to bio-loads than others, tank height/width, how often or how much the fish are feed, etc.
Today I don’t really follow any set rules. I let the tanks inhabitants and the water chemistry tell me when its full. I keep a close eye on water chemistry, algae growth, fish behavior, etc. I then balance that out with how often I know I will do water changes.
My current 29g setup.
6 Tiger Barbs
3 Emerald Cories
3 Cardinal Tetras
2 Dwarf Gouramis
2 Blue Rams
Some people would say that was 29 inches of fish, but I see it as 38 inches of fish and overstocked any way you look at it. However, the fish are happy, the plants are happy and the water chemistry is optimal as long as I maintain my 25% mid-week WC and my 40% weekend deep clean with WC.